1. Field
One or more embodiments of the present invention relate to a pixel circuit of a flat panel display device and a method of driving the same.
2. Description of Related Art
The thin-film transistor is composed of an active layer including a channel region, a source region, and a drain region, and a gate electrode that is overlapped with the channel region, and is insulated from the active region by a gate insulating layer.
The active layer of the thin-film transistor is generally made of a semiconductor material, such as amorphous silicon or a poly-silicon. However, when the active layer is made of amorphous silicon, mobility becomes low, so that the implementation of a driving circuit operated at a high speed is difficult. In addition, when the active layer is made of a poly-silicon, mobility becomes high, but threshold voltage is not uniform, so that a special compensating circuit should be added.
In addition, a method for manufacturing an existing thin-film transistor by using low temperature poly-silicon (LTPS) includes a process having high cost, such as laser thermal treatment, and it is difficult to control properties, making an application to a large size substrate difficult.
In order to solve the problems mentioned above, research using an oxide semiconductor as the active layer has been proceeding.
Japanese Laid Open Patent Application No. JP-A-2004-273614 discloses a thin-film transistor including as an active layer an oxide semiconductor that is composed of zinc oxide (ZnO) as a main material.
An oxide semiconductor composed of ZnO as a main material is considered as a stable material while being in an amorphous state. When using the oxide semiconductor as the active layer, the thin-film transistor may be manufactured at a low temperature by using the existing devices without purchasing a special additional device. Also, ion injection process may be omitted.
However, such devices may result in low reliability because electrical properties of the thin-film transistor having the oxide semiconductor as the active layer (hereinafter, referred to as “oxide thin-film transistor”) may be changed according to its configuration and processing condition. For example, when driving a constant-voltage or constant-current, deteriorating current properties and changes in threshold voltage can deteriorate electrical properties.
Therefore, when the pixel circuit or driving circuit of the flat panel display device, such as a liquid crystal display device or an organic light emitting display device, is formed using oxide thin-film transistors, image quality and performance may be deteriorated by non-uniform threshold voltages.